'Moose, probably not what you want to hear.... Front fender liners are high density plastic. Not likely to resonate. That's why they are used today (aside from reusting out). Rear duals have no liners. The under decking of the coach is all that's there. WBGO uses a thick plywood w/ aluminum sheet. Aluminum is covered with a protective film and faced down. Considering its thickness and internal structure in a double floor with a 'tween deck space, I dont think its going to transmit much road noise.
Trying to sound deaden an RV is going to be a mega challenge. They are a gopher hole of hidden spaces & hidey holes. Getting to them....well, it will not be easy and probably not effective.
Be careful with any material that may absorb moisture or is not treated to prevent molds. RVs are giant sweat lodges and moisture inside is always an issue...especially in hidden and un vented spaces. Getting toxic black mold would be the zombie apocalypse.
At the suggestion of a local 'sound merchant' who does custom sound systems in cars, I used stainless steel faced butyl tape for roof repairs and sealing. Easy to apply to curved surfaces and hard to get to places. Be sure to have it exactly where you want it. Once it touches it holds like a python.
I used it to MacGuyver sound damping on the Mach 8.
https://drive.google.com/file/d/0BzXeY3pXinO9ZXItem5QTVVLUDA/view?usp=sharinghttps://drive.google.com/file/d/0BzXeY3pXinO9QjR6aDlwSnVYNVk/view?usp=sharingSome folks have removed the interior paneling in the 'snout' and have 'insulated' it for better temperature control. They have used fiberglass or the silver bubble wrap material. The butyl tape might be an easier and better method.